Telecommunication devices typically alert customers to an incoming call by providing ringing signals, dial tones and busy signals. A centrally-located alternating current (AC) ringinig signal generator provides the necessary power to drive a ringer or audible tone source to create the desired acoustic signals. Today, electronic oscillators usually serve as ringing signal generators. The ringing signal generators are often located with the centralized switching equipment. Typical ringing signal generators produce an alternating voltage ringing signal having a constant frequency of about 16 to 30 hertz and a voltage of about 70 volts RMS.
Older telephone ringers include a series-parallel winding having an armature that drives a bell clapper. The ringing signal generator drives the winding. The winding requires a certain minimum drive current to provide sufficient magnetomotive force to drive the bell clapper. Because the winding has a nonliner response to drive current, the current waveform is often asymmetrical and tends to be peaked in one polarity direction. Also, due to reactive impedances in the winding, the drive current tends to be out of phase with the drive voltage. Moreover, operation of the winding requires bi-directional power flow to accommodate the reactance of the winding. More recent telephone ringers are completely electronic and, depending upon their design, may or may not require significant drive currents to operate properly.
In the familiar wire-based telecommunications systems, a central ringing signal generator typically provides power to the different ringers served by the systems over the wire pair normally used to provide voice communications. New telecommunications systems such as an Optical Network Unit used in fiber-to-home applications, however, require a different approach. Since fiber optic cables are unable to carry electrical current, the fiber-based systems are unable to employ the central ringing signal generator. A local ringer circuit is therefore required to generate the ringing signal to ring the telephones.
Four quadrant inverters are often employed to generate the local ringing signal. Currently, isolated, low power four quadrant inverters that process bi-directional voltage and current are sometimes implemented with a dedicated transformer, three independent active switches and a complex control scheme. While this topology allows the recycling of power in positive or negative voltage or current modes, the three switches should be operated based on instantaneous voltage and current information, requiring a complex control process. Complex custom logic circuits are typically required to control the individual active switches based on a polarity of the output voltage and a direction of the current flow. See, for example, Dhaval Dalal, "A Unique Four Quadrannt Flyback Converter," published by linitrode Corporation in "Power Supply Design Seminar" (1997), which is incorporated herein by reference.
Another telephone ringer topology is described in U.S. Pat. No. 4,866,587, which issued on Sep. 12, 1989 to Wadlington and is entitled "Electronic Ringing Signal Generator." The aforementioned reference is incorporated herein by reference. Wadlington discloses a converter topology that allows the recycling of the output power back to the input source (i.e., a bi-directional power flow). The converter of Wadlington, however, requires two inverters, a first inverter connected to a primary winding of the transformer and a second inverter connected to a secondary winding of the transformer. The first and second inverters each operate in only one direction.
Still another telephone ringer topology is described in Application Note AN-H35, entitled "High Voltage Ring Generator" published by Supertex Inc. (1997). The aforementioned reference is incorporated herein by reference. The Application Note describes the operation of a Supertex HV441 switchmode ring generator integrated circuit (IC). The ring generator disclosed by Supertex, however, not only requires a complex control circuit to operate, but also requires the power supply to develop high voltage DC buses that exceed a peak positive and negative output voltage of the ringer. Further, because the ring generator is embodied in an IC, the peak output voltage is limited by the inherent breakdown characteristics of the silicon process employed to produce the IC.
As telephones incorporate a number of advanced features, such as wireless operation, messaging, networking or video transmission capabilities, DC power of various voltages are also required to operate the electronic circuits associated with the advanced features. The fiber-based systems should thus ideally provide both the AC ringing signal to ring the telephones and the DC power required to operate the electronic circuits.
Accordingly, what is needed in the art is a converter topology that provides both AC and DC voltages and permits bi-directional four-quadrant power flow of the AC voltage, thereby overcoming the limitations of the prior art.